Tracking a shot with focal variation (zoom)
Sadly, this is not possible in Blender. There are other pieces of software which can do what you wan't:


Voodoo (thanks, someonewithpc)
Adobe after effects
Syntheyes
Mocha Pro
PFTrack (thanks, cgeaton)


Some of these are a little expensive, but they are all far superior to Blender's tracker in many ways.
Note: Mocha is the best matchmoving/roto/cleanup tool out their. by far. (No affiliation, promise), PFtrack also is very good at what it does, but it is still a point tracker.